Heat sink



FIG. 1 is a perspective view of a heat sink showing our new design;

FIG. 2 is a is an end elevational view thereof, the opposite end being a mirror image thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a side elevational view thereof, the opposite side being a mirror image thereof; and,

FIG. 5 is a bottom plan view thereof. 

The ornamental design for "heat sink", as shown and described. 